Method for collective production of microlenses at the tip of an optical fiber assembly such as a fiber tape

ABSTRACT

A method for the collective production of microlenses at the end of a set of aligned optical fibers. The method consists in heating the end of all the fibers by means of an electric arc in order to form the microlenses, the plane in which the ends of the fibers are situated being distant from the line of the hottest points in the electric arc in order to round their end homogeneously. Useful for making optical and optoelectric modules.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for the collective production ofmicrolenses at the end of a set of optical fibres, of the ribbon offibres type.

The present invention applies to optical and optoelectronic modulesamongst other things for optical telecommunications. It applies moreparticularly to the production of microlenses on optical fibres in orderto improve the coupling between optical and optoelectronic components.These microlenses are particularly well adapted to collective couplingwith active components in arrays, such as lasers, semiconductoramplifiers, VCSELs or photodetectors for example.

2. Discussion of the Related Art

In the literature a large number of articles are found having methodsfor the individual manufacture of microlenses at the end of fibres whichimprove the coupling between active components and monomode fibres. Thehistory of these microlenses is presented in the collection ofpublications “Microlenses Coupling Light to Optical Fibers”, Huey-DawWu, Frank S. Barnes, 1991, pp. 149-213: “Microlenses Coupling Light toOptical Fibers” IEEE Lasers and Electro-optics Society 1991 [1].

On the other hand, very few articles are found concerning collectivecoupling lenses.

The most recent articles report on combinations of lengths of fibres ofdifferent natures and the fashioning of a lens at the end of fibres, butalways in order to produce individual microlenses.

In fact, individual coupling lenses are known. Reference can be made tothe article by K. Shiraishi et al. (University of Utsunomiya, Japan) “Afiber with a long working distance for integrated coupling between laserdiodes and single-mode fibers”, Journal of Lightwave Technology, Vol. 13No 8, pp. 1736-1744, August 1995 [2], which presents a lens whoseworking distance is 160 μm for laser-fibre coupling losses of 4.2 dB andlateral and angular axial positioning tolerances respectively of 35 μm,2.6 μm and 0.8° for an additional loss of 1 dB. The results wereobtained for a laser emitting at a wavelength of 1.49 μm with a meantotal half-height divergence of 20.5° (that, is to say 34° at 1/e²).This is a length of fibre 1 without a core and with a hemispherical end,welded to a monomode fibre 1 whose core has been locally enlarged byheat treatment, as shown by FIG. 1.

In a more recent article, Shiraishi and Hiraguri “A lensed fiber withcascaded Gi-fiber configuration for efficient coupling between LDs tosingle-mode fibers” ECOC '98, 20-24 September, Madrid Spain, pp. 355-356[5], propose a new lens consisting of two lengths of monomode fibres, ofdifferent natures, whose focusing parameters are different, weldedtogether and to a monomode fibre by electric arc. A hemisphericalprofile is conferred on the end multimode fibre by means of an electricarc welder. Losses of 2 dB are obtained in front of a laser diodeemitting at 1.3 μm, whose total divergence in far field at half-heightof the maximum is 24.90×19.50 (that is to say 42.2°×33.1° at 1/e²). Theworking distance is 50 μm.

If the publications concerning individual fibre laser coupling lensesare numerous, those dealing with collective lenses intended formultichannel optical modules are more rare.

A method is known which consists in interposing an array of microlenses(not fixed to the fibre ribbons). By way of example, the coupling lensshown in FIG. 2 of G. Nakagawa and al. (Fujitsu Laboratories, Japan)“Highly efficient coupling between LD array and optical fiber arrayusing Si microlens array” IEEE Photonics Technology Letters, Vol. 5, No9, pp. 1056-1058, September 1993 [4], makes it possible to obtain4.8±0.3 dB by dynamic coupling between the array 4 of four lasers with atotal half-height divergence of 30° (that is to say 44° at 1/e²) and 4monomode fibres 2 ₁, 2 n by means of a matrix of silicon lenses. Thistype of coupling complicates the assembly steps, since it adds anadditional component to be positioned very precisely.

In 1996, J. Le Bris “High performance semiconductor array module usingtilted ribbon lensed fibre and dynamical alignment” ECOC '96 OsloTHc.2.3, p. 4.93, from the company Alcatel (AAR, France) proposes alensing method on a fibre ribbon which consists of chemically etching aribbon of monomode fibres and reworking the end of each fibre of theribbon by electric arc. With this method 3.6 dB of loss is obtained infront of an array of semiconductor amplifiers with ribbons misaligned by20×25° of total half-height divergence (that is to say 34×42.5° at1/e²). The wavelength is 1.55 μm.

The recommended solutions for the “lensing” of the fibres (the fittingof lenses at the end of fibres) which make it possible to obtain goodcoupling levels are not collective methods in the case of references [1]to [3].

In addition, the outside diameter of the 125 μm fibre is not maintainedall along the microlens, which poses a problem for the hybridisation ona silicon platform in precise positioning Vs and for precision ferrulefitting.

For the collective methods known at the present time, the couplinglosses are still too high. In addition, the use of discrete lensesdescribed in reference [4] requires several successive alignments, whichincreases the number of assembly steps compared with microlensesattached at the end of the fibre. The method described in reference [5]also imposes a very short working distance of less than 15 μm inaddition to the fact that it is complex.

SUMMARY OF THE INVENTION

The purpose of the present invention is to improve the coupling betweenan array of active elements and a set of aligned fibres of the fibreribbon type.

To this end, the invention relates to a method for the collectiveproduction of microlenses at the end of a set of aligned optical fibres,principally characterised in that it comprises a step of heating the endof all the fibres by means of an electric arc in order to form themicrolenses, the plane in which the ends of the fibres are situatedbeing distant from the line of hottest points of the electric arc inorder to round their end evenly.

The method according to the invention also has the advantage of beingcollective and therefore compatible with mass production, and with avery high performance.

According to another characteristic of the invention, the distancebetween the optical fibre ends and the line of hottest points is between850 micrometres and 950 micrometres.

Advantageously, the set of optical fibres consists of a ribbon.

According to a preferred embodiment of the invention, the ribboncomprises monomode fibres whose terminations comprise a length of silicawelded to a length of fibre with an index gradient, the microlensesbeing produced at the end of the lengths of fibres with an indexgradient.

BRIEF DESCRIPTION OF THE DRAWINGS

Other advantages and particularities of the invention will emergeclearly from a reading of the description which is made below and whichis given by way of non-limitative example with regard to the drawings,in which:

FIG. 1 depicts an individual coupling lens according to the state of theart,

FIG. 2 depicts a collective coupling lens according to the state of theart,

FIG. 3 depicts the outline diagram of the production method according tothe invention,

FIG. 4 depicts the diagram of a “lensed” ribbon of fibres according tothe method in accordance with the invention,

FIG. 5 illustrates a photograph of a “lensed” ribbon according to theinvention.

The method according to the invention consists in rounding the end of aset of fibres being in the majority of applications in the form of aribbon of fibre 10, by means of an electric arc welder, only theelectrodes of which are depicted at E1, E2, the ribbon 10 being placedfar from the line X of the hottest point so that the ends of the fibresof the ribbon are aligned at a distance d of around one millimetre(typically 900 μm) with respect to this hot point, in order to be placedon an isotherm. This makes it possible, unlike the “lensing” at the hotpoint of the electrodes E1, E2, to obtain a hemispherical shape which isnot only homogeneous over all the fibres of the ribbon, but also not tomodify the diameter of the fibres.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

According to a preferred embodiment, the method is applied to theproduction of hemispherical lenses with a microlens as described in thepatent EP 0 825 464 of the applicant.

The patent EP 0 825 464 relates to a collective microlens known asGRADISSIMO since it consists of lengths of multimode fibres with anindex gradient GRAD and silica SI welded successively together and to aribbon of monomode fibres MO, referenced 10 in FIG. 4.

The invention consists of collectively “lensing” the end of thismicrolens.

The losses are 2.5±0.05 dB in front of 60°×50° of total divergence infar field of 1/e² of the maximum intensity for working distances of100±5 μm, instead of 10.5 dB for 15 μm of working distance in front of acleft monomode fibre.

The losses are 1.4±0.05 dB in front of lasers of 21°×21° of totaldivergence in far field at 1/e² of the maximum intensity for workingdistances of 100±5 μm, instead of 3.2 dB for 15 μm of working distancein front of a cleft monomode fibre.

For this example application, the method consists in first producing themicrolens ribbon 10 known as “GRADISSIMO” by collective welding andcleaving of lengths of fibres with an index gradient and silica on aribbon of monomode fibres as described in the patent EP 0 825 464.

This ribbon is then placed, in the same collective welder as the oneused for producing the “GRADISSIMO” ribbon, typically at 900 μm from thenormal welding position on the optical axis. This is possible throughthe control (optional) which makes it possible to control the motors andthe arc of the welder by RS232 interface. An electric arc is then sent,and makes it possible to round the end of the lengths of fibres with anindex gradient as illustrated in FIGS. 3 and 4.

The diameter of the hemisphere depends on the electric arc/fibredistance and the electrode discharge current.

By way of example the Sumitomo type T62welder was used.

Then there was obtained collectively a ribbon of microlenses which ishereinafter referred to as “SUPERGRADISSIMO”, having a hemispherical endas illustrated in FIGS. 4 and 5 making it possible to improve thecoupling level in front of arrays of active components such as lasers,semiconductor amplifiers or photodiodes for example.

The fibres being situated far from the hot point, only the core of thefibre with a gradient index is melted so that the outside diameter of125 μm is maintained over the entire length of the microlens, includingat its end.

Here are a few example embodiments from a ribbon with 4 channels F1, F2,F3, F4:

EXAMPLE 1

The coupling of a “supergradissimo” ribbon was effected in front of aBRS laser with a wavelength 1.301 μm of 60°×50° of total divergence infar field at 1/e² of the maximum intensity.

The measuring conditions were as follows:

T°=21° C., polarisation current I=42 mA, reference power of the laser10,000 μW.

The results are illustrated by the following table:

Welding losses Channel silica/ Length Welding Length Radius (1 fibre =index of losses of index of Coupling Working one gradient silica(silica/ gradient hemisphere losses distance channel) (dB) (μm)monomode) (μm) (μm) (dB) (μm) F1 0.06 544.00 0.04 357.19 68 2.56 53.5 F20.02 546.50 0.03 358.00 68 2.59 54.3 F3 0.07 546.12 0.02 354.45 68 2.5553.3 F4 0.03 546.12 0.05 357.19 68 2.52 54.2

EXAMPLE 2

The coupling of a “supergradissimo” ribbon was effected in front of aBRS laser 1.310 μm of 210×21° of total divergence in far field at 1/e²of the maximum intensity.

The measuring conditions were as follows:

T°=22° C., polarisation current I=72.6 mA, reference power of the laser10,000 μW.

The results are illustrated by the following table:

Welding Reflect losses Length Radius ivity silica/ Length Welding of ofat end index of losses index hemi- of Coupling Working gradient silica(silica/ gradient sphere fibre losses distance Channel (dB) (μm)monomode) (μm) (μm) (dB) (dB) (μm) F1 0.05 275.00 0.05 279.00 82 −39.31.45 102.40 F2 0.04 275.00 0.02 281.00 80 −40.1 1.41 107.60 F3 0.03274.50 0.06 281.00 83 −41.9 1.38 107.80 F4 0.04 274.00 0.02 282.00 81−39.3 1.42 105.00

By way of comparison, because of its rounded profile, the reflectivitymeasured at the end of the fibre by means of a reflectometer of theWIN-R type from Photondtics is typically −40 dB instead of −14.7 dB fora cleft fibre.

In addition, the great working distance limits the power reinjected intothe laser diode after reflection on the fibre. This is very importantfor applications of the semiconductor amplifier type or lasers withexternal cavities for which the stray reflections interfere with thefunctioning.

A low-cost collective “lensing” method has just been described whichmakes it possible to improve the coupling between the arrays of activecomponents and ribbons of monomode fibres compared with the prior art(up to 1.5 dB of losses) for large working distances (up to 100 μm). Andthis in a homogeneous manner over ribbons of fibres, it being understoodof course that this is merely an example with 4 channels.

The applications of the invention in the field of telecommunications fitjust as well in distribution networks for their collective and low costaspect and in transmission networks because of their high couplingperformances and their low reflectivity level. The large workingdistances which they offer are an advantage for all applications, andare in fact less critical to position and greatly reduce the influenceof Fresnel reflections.

Reference can be made to the table annexed to the description whichillustrates results obtained for the radius of the spheres as a functionof the distance between the fibre ribbon and the hot point of theelectrodes E1, E2, the current sent to the electrodes in arbitrary unitsand the electrode discharge time. The margin indicated for each radiuscorresponds to the scattering of the values on the ribbon.

Distance ribbon/ Electrode Radius of Sample electrodes Current dischargehemisphere N° (μm) (μ.a) time (μm) 298 920 60 7 82 ± 5  297 920 60 7 60± 5  302 920 60 6 95 ± 5  288 910 60 5 110 ± 5  293 910 60 7 80 ± 5  285910 59 7 90 ± 5  277 910 60 4 75 ± 5  (3 impacts) 287 910 58 5 80 ± 5 (2 impacts) 295 900 60 6 82 ± 5  (ex. 2) 294 900 60 6 90 ± 5  290 900 607 85 ± 5  292 900 60 8 90 ± 5  291 900 59 9 85 ± 5  296 900 60 6 78 ± 5 (2 impacts) 287 890 56 5 110 ± 20  Test 890 55 3 100 ± 30  Test 850 63 575 ± 5  286 840 63 5 68 ± 0  (ex. 1) Test 830 63 5 70 ± 5  Test 730 63 5Not homogeneous Test 400 50 2 Not homogeneous Test 350 45 3 No roundedpart Test 300 50 2 Not homogeneous Test 200 50 2 Not homogeneous Test200 30 2 No rounded part Test  20 50 2 Large lenses not homogeneous

Beams are obtained with a hemispherical end of between 68 and 110 μmwith a homogeneity of ±5 μm on the 4 channels of the ribbon fordistances between hot point and ribbon ranging from 830 to 920 μm.Ribbons 286 and 295 are the subject of embodiments presentedrespectively in Examples 1 and 2.

What is claimed is:
 1. A method for the collective production ofmicrolenses at the end of a set of aligned optical fibres, characterisedin that it comprises a step of heating the end face of the end of allthe fibres by means of an electric arc, the end face of the ends of thefibres terminating short of a line of the hottest points of the electricarc and at a distance from this line in order to round all the fibreends homogeneously and simultaneously to obtain all the microlenses. 2.A method for the collective production of microlenses according to claim1, characterised in that the distance between the front face of the endsof the optical fibres and the line of the hottest points is between 850micrometres and 950 micrometres.
 3. A method for the collectiveproduction of microlenses according to claim 1, characterised in thatthe set of optical fibres consists of a ribbon.
 4. A method for thecollective production of microlenses according to claim 3, characterisedin that the ribbon comprises monomode fibres whose terminations comprisea length of silica welded to a length of fibre with an index gradient,the microlenses being produce at the end of the lengths of fibres withan index gradient.